Tuberculosis (TB) persists as a global health concern due to high prevalence of infection and drug resistance. More detailed knowledge of TB pathogenesis is needed to unravel novel approaches for prevention and treatment. Early antimicrobial mechanisms which are part of the innate immune response system are crucial for the outcome of the infection with Mycobacterium tuberculosis (Mtb). In this application we investigate a novel mechanism, how human macrophages (Mphi), the primary host cell of Mtb, inhibit growth of Mtb when they undergo apoptosis. Our preliminary data show that apoptosis of the Mphi infected with Mtb is associated with their capacity to exhibit strong anti- mycobacterial activity, whereas necrosis promotes extracellular bacterial growth. We further showed that virulent Mtb are able to avoid host Mphi apoptosis, whereas the attenuated Mtb strain H37Ra strongly induces apoptosis. We postulate that Mphi- apoptosis 1) restricts Mtb growth by sequestering the bacilli within apoptotic bodies and 2) packages Mtb for rapid and efficient elimination by freshly recruited phagocytes. Uptake of free Mtb is also associated with arrested phagosome maturation and unrestricted intracellular growth. We think that Mtb packaged in apoptotic bodies are eliminated more effectively by the defense systems of the Mphi. We will examine possible cooperative effector systems when uninfected Mphi are presented with Mtb contained in apoptotic bodies. We have also found that Mtb-induced Mphi apoptosis and associated anti-mycobacterial activity are dependent on the concerted action of tumor necrosis factor alpha, cytosolic phospholipase A2, and on intra-cellular Ca++ levels, but the specific role and function of these mechanisms is not understood. We will investigate the role of these mechanisms in induction of apoptosis and anti-mycobacterial activity and how attenuated and virulent Mtb differ in the activation of these processes. The goals, thus, are to 1) determine how a virulent Mtb induce apoptosis and anti- mycobacterial mechanisms and how virulent Mtb avoid it, 2) to find out how apoptotic Mphi block growth of Mtb and 3) to define the anti-mycobacterial mechanisms of naive Mphi after uptake of apoptotic infected Mphi.